1. Field of the Invention
The invention relates to intensified television cameras, particularly with respect to inhibiting blooming in low-light level television cameras (LLL/TV). Such television cameras include intensified charge coupled device cameras utilized as LLL/TV sensors.
2. Description of the Prior Art
LLL/TV sensors have long been known in the prior art. Such sensors include the photocathode type sensor as well as the charge pattern type sensor. The photocathode type sensor is exemplified by the image intensifier LLL/TV typically utilizing a charge coupled device focal plane array. The charge pattern type sensor is exemplified by the diode array type camera tube.
Generally, LLL/TV sensors inherently suffer from a phenomenon referred to as "blooming". Blooming typically occurs when viewing a dark scene and a very bright light, such as a bright light point, is introduced into the scene. As observed on the monitor, the bright point tends to spread obscuring adjacent image data. The blooming problem occurs at the first sensitive surface of the camera sensor encountered by the entrance light and all LLL/TV sensors traditionally include such a sensitive surface. Blooming is observed whenever the bright point of light significantly exceeds the dynamic range capability of the sensor. Blooming thereby renders the LLL/TV useless for night viewing of such scenes as harbors, where the anchorage is dark but worklights and shore lights are in the scene, and night observation of landing aircraft where the landing lights bloom to obscure the entire aircraft.
The blooming problem has been intractable since the development of LLL/TV. Some camera tubes, such as diode array types, exhibit less blooming than others, but heretofore there has not been any solution that inhibited the blooming phenomenon.
The cause of blooming has generally been assumed to be electron diffusion or charge diffusion. Electron diffusion may occur in photocathode type sensors and charge diffusion may occur in charge pattern type sensors. It is believed that such diffusion may occur in both the substrate of the sensor and in any space charge adjacent the substrate. The prior art has heretofore not developed any effective method to avoid such diffusion. All known prior art techniques are believed to use video gain processing in the circuitry of the camera. None are believed to resolve the problem at its source, i.e., the photocathode. Thus, it is believed that all blooming suppression techniques in the prior art use camera gain as the control parameter and are ineffective since they suppress desired detail along with the bloomed area. No known prior art method attempts to exert control of blooming in less than a full television frame interval (1/30 second), during which interval full diffusion can occur.